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Groundwater poses a number of challenges to underground infrastructure projects. Dewatering Specialist Dr Martin Preene, speaker at a forthcoming BGA event on this topic, discusses some the challenges and potential solutions used by civil engineers when working below groundwater level.
Excavation works for underground infrastructure such as deep basements or tunnels often encounter groundwater. Working below groundwater level can be difficult, as the excavation is at risk of instability and inundation. However, modern geotechnical techniques can allow groundwater to be safely managed.
In cities, space is at a premium. Increasingly, new infrastructure requires work below ground for deep basements, tunnels and other underground spaces. In London, Crossrail is a prime example of how new infrastructure is using below-ground engineering skills to fit new infrastructure beneath and around all the existing paraphernalia of a bustling city.
Civil engineering underground often hits a poorly understood but very real problem – groundwater. This is an especial problem when excavating in water-bearing soil (such as sands and gravels) or fissured rock (such as chalk or sandstone). Without suitable control measures, inflows of groundwater can flood excavations or tunnels, and can also lead to instability when the soils or rock around the excavation weaken and collapse – either locally or on a large scale.
Over the last few decades there have been several high-profile projects around the world that have been seriously delayed by groundwater problems. On the other hand, a much greater number of projects have successfully dealt with groundwater with no obvious problems. In the south of England, projects like High Speed 1 (known as the Channel Tunnel Rail Link during its construction in the early 2000s) and Crossrail have routes which successfully passed through multiple water-bearing strata, including crossing beneath the River Thames.
The secret is – not that it is a secret to geotechnical engineers – that there are several construction techniques that can be used to control groundwater.
The first thing that needs to be done to ensure that groundwater can be effectively dealt with is to gather information to understand the problem. This involves a site investigation, which might involve drilling and testing of boreholes, measurement of groundwater levels and tests to measure the permeability of the ground. Permeability is a measure of how easily water flows through soils or rocks. High permeability soils and rocks tend to be water-bearing and are typical of the conditions where groundwater can cause problems for construction projects.
The techniques used to control groundwater include:
Each of these approaches to groundwater control has different advantages and disadvantages, and is applicable only to certain soil or rock types. Selection of suitable groundwater control measures is one of the key aspects of the design of underground works.
Groundwater control in urban areas is the topic of a joint British Geotechnical Association and Engineering Group of the Geological Society event on 16 February 2016, taking place at Burlington House in London. The event will include discussion of environmental impacts from groundwater control, and dewatering case studies from the Crossrail project.
Visit the BGA's event listing page for further details, including a link to a live webcast.
Dr Martin Preene is a Chartered Civil Engineer with 30 years' experience of groundwater and dewatering problems. He acts as an independent groundwater consultant and advises on engineering projects worldwide. He is Vice Chairman of the British Geotechnical Association.